Ch. xxxvi] foliation and cleavage. 607 



dip at right angles to the axis of elevation, and are parallel to it in their 

 strike. No argument, thei-efore, can be drawn in favor of a common 

 origin from uniformity of strike in the slaty and foliated rocks ; for we 

 require, in addition, coincidence of dip ; and such is the variability of the 

 dip both of the slates and folia as to render this kind of proof very diffi- 

 cult to obtain. 



That the foliation of the crystalline schists in Norway accords very 

 generally with the planes of original stratification is a conclusion long 

 since espoused by Keilhau.^' Numerous observations made by Mr. David 

 Forbes in the same country (the best probably in Europe for studying 

 such phenomena on a grand scale) confirm Keilhau's opinion ; for the dip 

 of the Silurian and fossiliferous strata where they pass into the metamor- 

 phic agrees with the foliation of the contiguous gneiss, mica-schist, and 

 crystalline limestone. So also in Scotland Mr. D. Forbes has pointed out 

 a striking case where the foliation is identical with the lines of stratifica- 

 tion in rocks well seen near Crianlorich on the road to Tyndrum, about 

 8 miles from Inverarnon, in Perthshire. There is in that locality a blue 

 limestone foliated by the intercalation of small plates of white mica, so 

 that the rock is often scarcely distinguishable in aspect from gii^i^iss 

 or mica-schist. The stratification is shown by the large beds and 

 colored bands of hmestone all dipping, like the foila, at an angle of 32 

 degrees N. E.f 



In stratified formations of every age we see layers of siliceous sand 

 with or without mica, alternating with clay, with fragments of shells or 

 corals, or with seams of vegetable matter, and we should expect the mu- 

 tual attraction of like particles to favor the ciystallization of the quartz, 

 or mica, or felspar, or carbonate of Hme, along the planes of original de- 

 position, rather than in planes placed at angles of 20 or 40 degrees to 

 those of stratification. 



In Patagonia, a series of thin sedimentary layers of tufi" were observed 

 by Mr. Darwin to have . become porphyritic, first where least altered, 

 by a process of aggregation, small patches of clay appearing to be 

 shortened into almond-shaped concretions, which in those places where 

 they were more changed had become ci'ystals of felspar, having their 

 longer axes parallel to each other. In other associated strata, grains 

 of quartz had in like manner aggregated into nodules of crystalline 

 quartz.J 



May we not, then, presume that in rocks where no cleavage has 

 intervened, foliation and the planes of stratification will usually coincide, 

 as in all cases where cleavage happens (as in the writing-slates of the 

 Niesen on the Lake of Thun in Switzerland, containing fucoids) to agree 

 with the original planes of sedimentary deposition ? ]\Ir. Darwin con 

 ceives that " foliation may be the extreme result of the process of which 



* Norske Mag, Naturvidsk. vol. i. p. 11. 



f Memoir read before the Geol. Soc, London, Jan. 31, 1855. 



i South America, p. 149. 



